The present invention relates generally to the field of fluid flow control valves. More particularly, it relates to gate valves and, still more particularly, to split-gate valves employed in production and processing of oil and gas.
As industry finds and produces hydrocarbons trapped in ever-higher pressure environments, the demands on hydrocarbonhandling equipment constantly increase. Higher pressures and more extreme environments create stresses and strains on equipment that previously were unforeseen. New problems and unanticipated difficulties develop where, before, conditions were not so severe. One field experiencing ever-increasing demands for performance is fluid valve technology.
Gate valves are commonly used in oilfield process systems to control fluids at high pressures. A standard through-conduit gate valve includes a valve body having a flowway through which fluids pass, and a valve chamber intersecting this flowway on the interior of the valve body. A movable gate within the valve chamber can be reciprocated up or down across the flowway to regulate flow. With the gate in its upper, or open, position, for example, a flow port in the gate aligns with the flowway to permit fluids to flow through the valve. In the lower, or shut, position, the solid portion of the gate blocks flow through the flowway. The gate is reciprocated within the valve chamber usually by means of a stem, which in turn is actuated by various other means external to the valve, e.g., manual, electric, hydraulic or pneumatic actuation means.
One prior art gate valve has a gate split into separate, symmetrical gate members. The gate members along a plane aligned with the longitudinal axis of the valve stem. The construction of the valve also allows the valve chamber to be in communication with the high pressure in the flowway while this gate is in the open position. The valve chamber then retains the elevated pressure after the gate closes. The pressure thus trapped in the valve chamber then communicates with a space between the gate members of the closed split gate. This trapped pressure presses outward on the gate members, thereby ensuring sealing engagement with the valve body. The valve chamber pressure also forces sealant material to flow through distribution channels to provide sealant to the appropriate seal locations. The valve chamber pressure thus provides the motive force for pressurizing the seals that maintain the leak-tight integrity of the valve when the gate is shut.
This type of valve, requiring a pressurized valve chamber for optimum sealing operation, often has a pressure differential between the valve chamber and at least one portion of the sealed flowway, usually the downstream portion, due to a partial or total loss of pressure in that flowway portion after the valve is closed. In high-pressure valves, especially, this pressure differential can be quite considerable, up to 15,000 to 20,000 psi, or more. In valves of this split-gate design, with gate members free to move against their respective seats in response to this chamber-to-flowway differential pressure, the differential actually helps to maintain the leak-tight seal. The high pressure on the chamber side of each gate member, being greater than the pressure on the flowway side, presses the gate member more tightly against seats in the valve body. With the gate member pushed more firmly against these seats, the flowway is more effectively sealed. The pressure within the valve chamber, then, serves the dual purpose of providing a seating force to help maintain sealing contact between the gate members and their seats, as well as pressurizing the seat seals themselves.
The valve chamber pressure can sometimes create a problem in a split-gate valve. As previously described, the gate members are free to spread apart and press against their respective valve body seats in response to a differential between pressure inside the valve chamber and pressure outside, in either of the exterior, sealed portions of the flowway. As noted above, such a pressure differential between the valve chamber and the downstream, pressure-depleted portion of the closed flowway is desirable for maintaining the pressure seal. Normally, though, the pressure in the upstream portion of the flowway remains the same as the pressure trapped inside the valve chamber, so there is no pressure differential across the upstream gate member. On occasion, however, there may be very quick depletion of pressure in a connected upstream system. This depletion makes the upstream flowway portion similarly lose its pressure very rapidly. Consequently, the valve chamber retains its original higher pressure, while both upstream and downstream portions of the flowway have much lower pressures. To open the valve, the gate must be reciprocated back to the upper position. The seating force arising from the differential in pressure between the valve chamber and the flowway, while helping to keep each gate member pressed against its seat, also increases friction between the gate member and the seat. This friction resists reciprocating motion of the gate. The friction force so resisting the gate motion is termed "gate drag."
The gate valve is then said to be "pressured-locked." When only the downstream portion of the flowway is pressure-depleted and the valve chamber retains higher pressure, only the downstream gate member experiences gate drag. In the pressure-locked condition, however, the valve chamber pressure forces both the downstream and the upstream gate members against their respective seats. Greater friction acting on both gate members heightens the valve's total gate drag. The magnitude of the gate drag, moreover, increases in proportion with the amount of the pressure differential. High-pressure valves demand an even greater gate-opening force in the pressure-locked condition than do lower-pressure valves.
The increased gate drag can tax or exceed the capabilities of the gate-actuation means, including the stem and whatever external means work to move the stem. The stem size could be increased, but at a cost of increasing all the associated parts of the valve. Economy and space also limit the feasible size of the gate actuation means. It can be seen, then, that a need exists for lessening gate drag at a minimum expense, without increasing the size or complexity of valves or actuators. As greater pressures are encountered, this need will only grow more acute.